When did the myth about mermaids start?

Answers

This is neat!
The oldest mermaid stories are from Assyria around 1000 BC. Its a typical myth about a goddess who loved a human and ended up killing him. In her shame, she jumped into a lake and turned into a half-human half-fish. Her name, as a mermaid, was Atargatis and (in Greek) Derketo.

Another popular mermaid was Alexander the Great’s sister, Thessalonike, who was rumored to have turned into a mermaid after her death. She would roam the Aegean Sea and ask sailors if Alexander was still alive.

So to answer your question, mermaid myths originate in the Near East, circa 1000 BC.